Foreword

Through a few of my previous articles we’ve already learnt quite a lot regarding different mains voltage guard equipment. We have studied a couple of circuits like mains circuit breaker, surge protector and how they help to stop and correct the relevant mains voltage abnormalities.

However, modern electronic devices like TVs, music systems, computers mostly come with built in voltage regulators which are able to tackle voltage fluctuations quite reliably and very seldom do they fail. Moreover brief voltage failures and quick successive voltage interruptions don’t bother them too much.

But domestic electrical gadgets which involve motors, especially in the form of compressors, for example as in refrigerators and air conditioners, can be particularly prone to abrupt or sudden voltage switching. Also if there’s an interruption in the power supply, preferably such appliances need to be shut down for at least a couple of minutes and then switched ON just to allow proper settling of the compressor fluid.

Since it’s not practically feasible to continuously monitor such malfunctions, a suitable automatic device if installed for the purpose can definitely prove very handy. So what do we exactly expect the device to do? Off course, simply to switch off the refrigerator or similar load during brief power failures (even instantaneous power interruptions) and switch it ON back exactly after a set period of time irrespective of the mains power situation.

The proposed design of refrigerator appliance compressor failure protector circuit is straightforward, complies with the above features. Once installed, it will accurately carry out the required safety measures and never allow any malicious voltage disorder to break in.

Let’s discuss its circuit functioning.

slide 2 of 2

Circuit Description

WARNING: THE CIRCUIT MAY HANG WITH DANGEROUS AC MAINS POTENTIALS WHEN POWERED ON, SO OBSERVE UTMOST PRECAUTION AND CARE DURING THE BUILDING AND THE TESTING PROCEDURES.

The IC 4060 as also discussed in many of my previous articles yet again proves how versatile it is in its applications.

It’s definitely the heart of the circuit and solely completes the entire control functions and time delay operations.

As shown in the figure alongside, we see that the IC is configured in its usual and the most standard mode.

C5, R3 and P2 together as well as individually form the time delay determining components. P2 may be adjusted to set the delay-ON time period anywhere from 1 to 30 minutes, for the present application it may be set for obtaining a delay of around 1 minute.

The circuit is powered through a standard transformer/bridge power supply circuit which is further regulated using a 7805 voltage regulator IC.

Pin # 12 of the IC is its reset pin and the IC remains functional (oscillates) as long as this pin is kept at a logic low potential or ground.

Initially when power is switched ON, C1 resets the IC so that it starts counting from zero onwards and not from any intermediate value and also at that very instant T1 on receiving a base bias conducts to ground pin 12 of the IC, initiating the counting process.

The IC starts counting, and the moment the set time delay period elapses, its pin #3 goes high, triggering the Triac. At the same time, the high or the positive status from pin #3 reaches pin #11 also via D6, immediately seizing the counting process so that the IC “freezes" at that position.

Since the load is connected via the triac, during this time it remains switched ON and operates as long as the input mains power is stable.

Now suppose due to some power disorder the mains voltage gets interrupted (even for a second), the base voltage to T1 correspondingly gets inhibited so that it stops conducting for that fraction of a second. Such occurrences instantly reset the IC, switching off the triac and the load.

Even if the mains voltage is restored at the very next second the load remains switched OFF until the circuit has finished counting and the set delay period has elapsed.

The above feature very effectively diffuses any possible chance of developing a spike/transient and also makes sure that the circulating fluid inside the refrigerator receives enough time to settle before restarting. The system thus very faithfully serves the function of a perfect refrigerator appliance compressor failure protector.